Many semiconductor devices are made of silicon. Silicon forms a stable oxide layer, silicon dioxide, on its surface when exposed to the atmosphere, sometimes referred to as "native" oxide. In previous years, when semiconductors had larger dimensions, this "native" oxide layer (9 to 15 angstroms) was an acceptable base for growing high quality oxides with a thickness around 1000 angstroms. Therefore, the native oxide could be left in place. Recently, however, the overall dimensions of semiconductors have been getting smaller. The purposefully grown oxide layers for use, for example, as FET gates, have been reduced to a thickness around 200 angstroms. At this thickness, the "native" oxide becomes an inhibitor to device quality and must be completely removed, prior to the growth of a high quality oxide layer to serve as a gate.
Numerous attempts have been made to develop a method and apparatus for removing oxide completely from a silicon substrate such as a semiconductor wafer without leaving undesirable residues on the substrate. More specifically, attempts have been made to develop a method and apparatus which could perform a series of cleaning cycles, with near consistent results, from one substrate to the next. However, these attempts have been unsatisfactory due to a number of problems.
For example, one problem arises when attempting to remove oxide from a silicon substrate with cleaning solution, such as Hydrofluoric acid ("HF"), in its liquid form. This is done by immersing the substrate in an aqueous HF bath. The surface of the substrate becomes hydrophobic and water droplets form on it. These water droplets create a major contamination problem. When the water droplets dry, they leave a residue of inorganic substances.
A further problem arises with attempts to utilize organic material such as isopropyl alcohol or methyl alcohol, to "get" the water droplets that remain from the above process, and thus alleviate the residue problem. Namely, a potentially harmful organic film is left on the substrate's surface.
Another attempt to remove oxide from a silicon substrate has been to use anhydrous HF. With this method, however, there is no substantial reduction of the oxide thickness. Therefore, this method does not achieve the desired oxide removal.
A further attempt at removing oxide from a silicon substrate has been to use energetic ion bombardment from plasma discharges. The principal difficulty here is that the discharges often affect the materials of construction of the chambers containing such discharges. This results in unwanted deposition of chamber constituent materials on the substrates which further results in contamination.
Yet another method is to use a vapor created from evaporating hydrous HF. An uncontaminated oxide-free surface results from this process because the substrate is stabilized by the presence of adherent hydrogen atoms on its surface. The difficulty presented with this method, however, lies in the inability to produce a consistent concentration of HF/water vapor since the mixture is azeotropic. That is, it is difficult to introduce an HF/water vapor of uniform stochiometry to the substrate. Previous methods of introducing the HF vapor and water vapor included the steps of bubbling a carrier gas through a liquid feed stock, then entraining the cleaning vapor with the carrier gas to the process chamber. The azeotropic nature of the feed stock would result in a continuously varying and unpredictable concentration of HF/water vapor in the carrier gas. This would lead to inconsistent and unpredictable results as far as cleaning the substrate is concerned.
An example of such a method is found in Tanaka, et al., U.S. Pat. No. 5,288,333, which includes a wafer-cleaning method and apparatus wherein a cleaning solution is caused to evaporate at a temperature below its boiling point. The cleaning vapor produced is applied at a temperature above its dew point to a wafer, such as a semiconductor wafer. It is difficult to consistently control the mixture or stochiometry of the HF/water vapor and carrier gas as it enters the process chamber to remove the oxide from the wafer. Moreover, it is more costly to perform this process since the Tanaka apparatus includes intricate controls and systems.
In addition, it is difficult to uniformly control or regulate the exhaust rate, and therefore the flow rate, of the mixture as it enters, reacts with the substrate within, and exits the process chamber.
Therefore, there is a need to perform a method that removes oxide from silicon substrates with substantially uniform etching and cleaning results from one substrate to the next (cycle to cycle) in a series of cleanings. There is a further need to perform this method using conditions which provide for consistent stochiometry, from cycle to cycle. Finally, there is a need to perform this method in a simpler, and less costly apparatus system and where the exhaust rate (or flow rate) is controlled or regulated to allow for repeatable, substantially uniform etching and cleaning results.
The copending, commonly assigned United States Patent Application of Gary Hillman, entitled "Flow Controlling Manometer", filed of even date herewith, (the "Manometer Application"), the disclosure of which is incorporated by reference herein, provides generally a manometer for use as a flow rate regulator or controller.